With multiple objects selected, how can I tell which keyframes in the timeline relate to which object?
As you have noticed, the timeline is good for


scrubbing,
playback settings and options and
manipulating the keyframes of a single or very few objects.


You need to switch to another animation editor type: the dopesheet or the graph editor.




  The Dope Sheet gives the animator a birds-eye-view of the keyframes inside the scene.




The dopesheet can show


the object's name (1),
the object's action (2) (if you have set keyframes you will have exactly one action)
the keyframe groups name (3) (keyframes for the same object are groups)
the names of the individual animation curves (fcurves) (4)


You can also toggle the Only Selected option (5), which will only show the keyframes for the selected objects.

The dopesheet provides you with a good overview for your keyframes. It is not only possible to manipulate the time property of keyframes (on which frame they are set) but also their values.


Click to enlarge.

If you wish to view the keyframes as individual curves or have finer control over their handles, you can switch to the graph editor.